1. Field of the Invention
This invention relates to processes for recovering oil from a reservoir by the injection of miscible drive fluid into the reservoir from longitudinally spaced injection wells. More particularly it relates to the recovery of oil from subterranean reservoirs having strata of differing permeabilities by utilizing a miscible drive fluid selected to have a density such that the drive fluid tends to be forced by gravity into the strata having the lowest permeability.
2. Prior Art
In most oil recovery processes, wells are drilled to subterranean formations containing the oil, and after the formation is thus penetrated, the oil is forced from the petroleum-bearing formation, or reservoir, to a wellbore as a result of the natural gas pressure or hydraulic force within the reservoir, thence to the surface of the earth also by natural pressure or by the use of pumps. Over a period of time, for example, after a substantial volume of oil or gas has been removed from the formation, the natural forces which had been initially driving the oil to the wellbore gradually decrease, sometimes causing the natural flow of oil to stop. However, in most such instances, more oil remains in the reservoir after its initial natural producing energy has declined or stopped than has been recovered from the reservoir. Rather than allow this oil to remain in the ground, it is common to initiate secondary recovery methods like waterflooding, or even tertiary recovery methods to assist in the recovery of the remaining oil. Among these enhanced oil recovery methods are those which have been termed "miscible drive" methods.
In miscible drive methods, a fluid phase is injected into the reservoir and passed through its subterranean formations, more or less horizontally from an input well to one or more output wells. For example, it is well known to inject a fluid which is miscible with oil into a reservoir, and then to inject a driving gas into the reservoir to move both the miscible fluid and the oil towards an output well. In U.S. Pat. No. 3,047,063, it is taught that where an oil reservoir includes zones of different permeability, the oil can be uniformly removed from the different permeability strata by the steps of injecting water into the reservoir, which water is preferentially absorbed into the more permeable zones, followed by injection of an oil miscible fluid, which is in turn followed by the injection of a driving gas which gas is miscible with the miscible fluid phase. The water absorbed into the permeable zone slows the rate of advance of the miscible fluid and driving gas in the more permeable zones, so that the rate of advance is substantially the same in both the more permeable and the less permeable strata. It is thus seen that this recovery method requires three separate injections of different fluids and gases into the well, and for some situations, if too great a pressure is applied, the gas will liquify and reduce or lose its driving force. In other systems, such as in U.S. Pat. No. 3,363,684, it is taught that in secondary recovery it is useful to inject water substantially more dense than the crude oil below the oil-water interface. Thus, for example, injecting dense salt water displaces and floats the less dense crude oil upwards out of a production well. It has been found in practice that this technique leaves a great amount of oil behind due to the fact that salt water is not miscible with oil.
In yet another system, U.S. Pat. No. 3,687,198, it is taught that when carbon dioxide is liquified under pressure, and injected into a reservoir near the oil-water contact under conditions of pressure and temperature such that the liquified carbon dioxide has a density greater than the reservoir oil, but less than encroaching aquifer water, the carbon dioxide is driven substantially upward through the reservoir by the aquifer or injected water. The carbon dioxide then miscibly displaces the oil, driving it to an output well from which it can be removed at the earth's surface. However, this process does not deal with the problems encountered when the reservoir strata includes different permeability materials. In yet another process, as taught in U.S. Pat. No. 3,661,208, carbon dioxide liquified under pressure is continuously injected into a reservoir and maintained at temperature and pressure conditions such that it substantially equals the density of the reservoir oil, thereby avoiding gravity segregation between itself and the oil, and thereby, theoretically, uniformly driving all of the reservoir oil before the miscible liquid carbon dioxide fluid. However, this system does not deal with those all too common situations in which strata of varying permeabilities are encountered within the reservoir. This latter situation results in the less-permeable strata being more or less bypassed by the injected pressurized fluid which seeks the path of least resistance through the more permeable strata, regardless of the fact that the fluid matches the density of the oil in both the permeable and non-permeable layers.
It is therefore seen, that while methods of extracting oil from a reservoir containing strata of varying permeability requiring multiple injections of fluid are known in the prior art, and that methods of injecting a single fluid of a density selected to match, and thereby to displace or drive oil, are also known in the prior art, none of the prior art techniques are suitable or useful for the efficient recovery of oil from a reservoir including strata of varying permeability by a process of injecting a single miscible drive fluid into a reservoir.